Scanning analyzer for determining characteristics of an ionized plasma



J. F. HOLT 3,519,927 SCANNING ANALYZER FOR DETERMINING CHARACTERISTICS July 7, 1970 OF AN IONIZED PLASMA 2 Sheets-Sheet 1 Filed Sept. 5, 1968 QMQQQUNM INVENTOR. 1/17/76! E 1/0 7' BY $4 an 197'70 1V6! nnvr y 1970 J. F. HOLT SCANNING ANALYZER FOR DETERMINING CHARACTERISTICS OF AN IONIZED PLASMA 2 Sheets-Sheet 2 Filed Sept. 5, 1968 g N R7 7 01. I H 6 73 Mo a/Wm M J l? m v w e a mum s m. -2 f Z M 4 v m awn v0 Y i l B M mu 2 T M q. z t

United States Patent Office 3,519,927 Patented July 7, 1970 US. Cl. 32424 Claims ABSTRACT OF THE DISCLOSURE Apparatus for measuring and displaying plasma characteristics such as current density and electron temperature over the cross section of the plasma stream. A rotating apertured scanning disc intercepts the plasma stream and scans it along parallel lines. The plasma particles passing through the apertures of the scanning disc are received by a collector electrode producing a current flow in the electrode circuit which varies in accordance with the current density distribution in the plasma stream. By varying the collector voltage the EI characteristic for any point may be obtained, from which the electron temperature may be derived. Profiles of both current density and electron temperature are displayed in a gray scale representation on the screen of a kinescope the scanning beam of which is synchronized with the scanning disc.

BACKGROUND OF THE INVENTION The invention relates to the study of gaseous electrical discharges or plasma and in particular to apparatus and techniques for measuring the characteristics of plasma streams. In the past the characteristics of gaseous discharges have been investigated by the use of conductive probes inserted into the particle stream, the probe current being analyzed to give the desired information. US. Pat. 3,207,982 to Rose is an example. Where profiles of a particle stream for particular parameters such as current density and electron temperature are desired it is difiicult to obtain high resolution with probes since the elemental area of the plasma cross section being investigated at any one position of the probe is not well defined. Further, the relatively rapid scanning of the cross section of a plasma stream is not easily accomplished with a moving probe.

SUMMARY OF THE INVENTION The purpose of this invention is to provide relatively simple means having good resolution for deriving and displaying a cross section or profile of a plasma stream with respect to a particular characteristic of the plasma such as current density or electron temperature. This is accomplished by intercepting the plasma stream with a rotating scanning disc having small apertures which scan one at a time across the plasma stream along adjacent parallel lines. The plasma particles that pass through the scanning apertures are received by a collector electrode. The resulting electrode current flowing through a load resistor produces a voltage proportional to the current density of the plasma stream at the point in its cross section where the scanning aperture is located at the instant. This voltage may be used to intensity modulate the beam of a kinescope, the sweep of which is synchronized with the scanning disc, to display the current density distribution in the plasma cross section, or, in a manner explained later, the collector potential may be continuously varied and the resistor voltage further processed to produce a signal representative of electron temperature which is then used to intensity modulate the kinescope beam to display the electron temperature distribution in the plasma cross section.

This technique provides good resolution because the scanning aperture provides well defined elemental areas in the plasma cross section, the resolution increasing of course as the size of the apertures is decreased.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view illustrating a scanner in accordance with the invention installed in a plasma chamber for analyzing a gaseous discharge;

FIG. 2 is a partly sectional view taken at the planes indicated by line 22 in FIG. 1;

FIG. 3 is a cathode ray tube display system for the scanner of FIGS. 1 and 2; and

FIG. 4 illustrates collector current as a function of collector voltage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1 and 3, 1 represents a sealed plasma chamber in which low pressure arcs may be formed and in which the plasma scanner for the study of such arcs in accordance with the invention is situated. The chamber is maintained at a suitably low pressure, for example 35 l() torr, by vacuum pump 3 to which it is connected by a conduit 4. The chamber is equipped with two removable circular end plates of metal, plate 5 at the right end of the chamber supporting the cathode of the arc circuit and plate 7 at the left end supporting the ring anode 8 of the arc circuit together with the components of the plasma scanner.

Cathode 6 is a hollow cylinder of a refractory metal supported in an electrically and thermally conductive manner by a liquid cooled cathode support 9 which in turn is supported in end plate 5 by an insulating sleeve 10. A suitable gas, such as argon, is admitted to the hollow cathode through tube 11.

The ring anode 8, made of a suitable metal such as copper, is hollow for liquid cooling and is supported by metallic liquid coolant tubes 12 and 13 best seen in FIG. 2, which is a partly sectional view taken along the planes represented by section lines 22 of FIG. 1. These tubes have hollow metallic right angle extensions 14 and 15 from which the ring anode is supported by metallic tubes 16 and 17. The coolant tubes and these extensions also serve to connect the ring anode to the grounded end plate 7.

The cathode 6 is established at a negative potential relative to the anode 8 by means of direct current source 18. The resulting electric field between the anode and cathode causes negatively charged particles at the cathode to be accelerated toward the anode to form the plasma stream 19. An axial magnetic field H, established by direct current flow in fixed coils surrounding the plasma chamber, opposes any divergence of the charged particles and confines them to a stream of cross section small enough to pass through the ring anode 8.

The scanning disc 20, made of an insulator capable of withstanding the plasma such as boron nitride, is supported by shaft 21 which in turn is rotatably supported by end plate 7. The scanning disc is positioned to intercept the plasma stream 19 and is provided with a number of scanning apertures of the same size, such as aperture 22, having equal angular spacings and uniformly decreasing radial distances from the center of rotation of the disc in the manner of early television scanning discs. The appearance of a disc with 8 apertures at 45 spacings is illustrated as a part of the system in FIG. 3. As indicated in this figure, the design must be such that the angle subtended at the center of rotation by the cross section of the impinging plasma stream 19, shown dotted in FIG. 3, is less than the angular spacing of the apertures in order that only one aperture will scan across the stream at any one time. With this arrangement the area is scanned along successive parallel arcuate paths. To completely scan the cross-sectional area, the sum of the aperture diameters should equal the diameter of the cross section. For example, referring to FIG. 3, a plasma stream 19 of one-inch diameter could be completely scanned in one revolution by a disc having 8 apertures, as shown, of /8 inch diameter. The size of the apertures is of course determined by the resolution desired and in turn dictates the number of apertures required and the minimum disc diameter.

The charged particles that pass through the apertures of the scanning disc are received by a conductive collector electrode 23 situated behind the disc and opposite the area of the disc that receives the plasma stream. This electrode is supported by a conductive rod 24 held in an insulating sleeve 25 in end plate 7. Those particles that do not pass through an aperture are reflected from the scanning disc and eventually are collected by anode 8. In order to prevent particles from passing around the edge of the scanning disc to collector electrode 23, a baffie 26, which may be made of the same material as the scanning disc and which has a central opening 27 to permit passage of the plasma stream, is positioned just in front of the scanning disc. As may be seen in FIG. 2, the baffle is supported by coolant tubes 12 and 13 which pass through edge notches 28 and 29, collars 30 and 31 serving to hold the baffle against right angle extensions 14 and 15.

The collector electrode 23 is connected through resistor R to a direct current source 32 which holds it at either a higher or a lower potential than that of anode 8 by an amount E, the relationship between collector current I and the voltage B being as shown in FIG. 4. The charged plasma particles passing through the scanning aperture and striking the electrode 23 cause a current to flow through R. If R is much smaller than the plasma resistance then the voltage e developed across R is proportional to the plasma current density at that point in its cross section at which the scanning aperture is located at the time. In the most elementary application of the scanner, the scanning disc may simply be rotated manually to position a scanning aperture at the poit in the plasma cross section it is desired to investigate. The plasma current density I at that point will then be shown by a suitable indicator 33. Also, by varying E and noting the values of I for each value of E the E-I characteristic at that point may be derived. From this relationship, the electron temperature at the point may be derived using the theory of Langmuir as understood in the art and as will be explained more fully later in connection with FIG. 3. If E is varied linearly with time, as by a suitable constant speed drive 34, and if indicator 33 is an X-Y recorder running at constant speed, the EI characteristic will be drawn by the X-Y recorder. Again-using an X-Y recorder running at constant speed, if E is held constant and scanning disc 20 is rotated at constant speed, as by a constant speed motor 35 coupled to shaft 21 through pinion 36 and gear 37, a series of consecutive graphs will be drawn by the X-Y recorder representing the current density distributions along the paths of the scanning apertures across the cross section of the plasma stream.

Where it is desired to produce a gray scale representation of the current density distribution or the electron energy distribution over the plasma stream cross section, apparatus such as shown in FIG. 3 may be employed. Referring to this figure, when switch S1 is actuated to the left a video signal proportional to current density is produced on line 38, and when actuated to the right a video signal proportional to electron temperature is produced on this line. These signals are applied to the beam intensity control electrode 39 of cathode ray tube indicator 40 for controlling the intensity of the spot of light produced on its screen by its electron beam. With S1 actuated to the left for the current density display, a constant voltage E is applied to collector electrode 23 through R and the output of differential amplifier 41 is coupled through contrast control 42 to the input of video amplifier 43. Amplifier 41 amplifies the difference between the upper end of R and the voltage at its adjustable contact, i.e. the voltage drop across R, so that its output is proportional to the current densiy I, i.e. the total current in the elemenal area of the plasma cross section defined by the scanning aperture. With S1 actuated to the right, a linearly varying sawtooth of voltage V, produced by sawtooth generator 44, is applied through R to the collector electrode 23 and the output signal I of amplifier 41 is applied to a log converter 45 which produces an output signal Log I. This signal is applied to operational amplifier 46 which produces an output signal proportional to the derivative of its input Log I with respect to V or d (Log I) /dV. Actually circuit 46 produces the derivative with respect to time, but since V varies linearly with time it is also with respect to V. In accordance with the Langmuir theory, the peak value of this derivative is inversely proportional to electron temperature. This signal is applied through contrast control 42 to the input of video amplifier 43 and thence to control grid 39, producing a spot of light on the screen of indicator 40 proportional in intensity to its peak value. The frequency of the voltage sawtooth V should be high enough relative to the scanning speed that several cycles occur before the scanning aperture has moved appreciably. The amplitude threshold of indicator 40 may be adjusted at potentiometer 47.

In order to have the beam of cathode ray tube 40 scan across its screen along arcuate paths in the same manner that the apertures of scanning disc 20 move across the plasma cross section, a circular radially deflected sector sweep similar to that employed in a sectored PPI type of display is used. Preferably, the center of the circular sweep is displaced from the center of the screen to a point such as point 48, or to a point outside the area of the screen if necessary, to provide a sufi'iciently large image area 49. This displacement is accomplished in conventional manner by the centering circuits of the vertical and horizontal drive networks 50 and 51 which energize the vertical and horizontal deflection coils 52 and 53.

Both the radial displacement of the beam of tube 40 from the center 48 and direction of this displacement are controlled by sine-cosine resolver 54 having stator windings 55 and 56, apart, and rotor winding 57. Rotor winding 57 is energized from the secondary of transformer 58 through tap changing switch 8-2, the primary of this transformer being energized from alternating current source 59. The outputs of the stator windlugs 55 and 56 are rectified by rectifiers 60 and 61 to produce sweep inputs to the vertical and horizontal drive circuits 50 and 51. The application of voltage to winding 57 radially displaces the beam of tube 40 from the center 48 by an amount proportional to the voltage. The direction of this displacement depends upon the angular relation of winding 57 to windings 55 and 56, and rotates about center 48 when coil 57 is rotated. Therefore, by applying a constant voltage to winding 57 and rotating this winding in synchronism with scanning disc 20, the beam of tube 40 may be made to trace a circular path across the screen that is centered on point 48 and corresponds exactly to the circular path of a scanning disc aperture across the plasma cross section. Thus, under the conditions illustrated in FIG. 3, in which winding 57 has maximum coupling to winding 55, zero coupling to winding 56, and maximum energization since S-2 is on the highest voltage tap, the beam of tube 40 has its maximum displacement from center 48 and the direction of the displacement is normal to the axis of vertical deflection coil 52 or along line 62. Therefore, the beam rests at point 63 which corresponds to the position of the outermost aperture 64 of scanning disc 20 just prior to scanning across the plasma cross section. If coil 57 is now rotated clockwise in synchronism with disc 20 the beam point 63 travels in an arcuate path 85 across the screen corresponding exactly to the arcuate path taken by aperture 64 across the plasma cross section.

In order for the beam to scan a similar arcuate path across the screen for each of the apertures of disc 20, winding 57 is mechanically driven from shaft 21 of the scanning disc through gears 65 and a repeating sector drive 66. The latter comprises an incomplete pinion 67 and a sector gear 68 which is directly coupled to the rotor winding 57 of the resolver. The ratio of gears 65 is such that pinion 67 rotates through 360 for a rotation of the scanning disc through the angle between apertures. In the illustrated case this angle is 45, requiring a gear ratio of 8:1. Pinion 67 and sector 68 have the same ratio so that sector 68 and rotor winding 57 are driven at the same angular velocity as scanning disc 20. The number of teeth omitted from pinion 67 is made such that sector 68 is released by the pinion after slightly less than 360 of rotation of the pinion from the pickup position in which it is illustrated. The purpose of the slightly early release is to allow time for the rapid counterrotation of the sector by spring 69 to its pickup position against stop 70 by the time the pinion has completed exactly 360 of rotation.

The above-described cycle of operation of sector drive 66 is repeated during each 45 of rotation of the scanning disc, each cycle of operation starting as each aperture of the disc passes over the angular position indicated by line 89. The quick return of sector 68 to its start position against stop 70 at the end of each cycle advances tap changing switch S-2 clockwise by one position through ratchet wheel 71. Therefore, starting from the position illustrated in FIG. 3, rotation of scanning disc 20 clockwise through 45 causes aperture 64 to scan along a circular path across the plasma cross section and the beam of tube 40 to move in synchronism with the aperture along a circular path 85 across the screen from point 63. Similarly, for the next 45 of rotation of disc 20, aperture 72 moves from position 89 across the plasma stream and the beam of tube 40 similarly moves from point 73 along a circular path 74 of slightly shorter radius due to the fact that S-Z has now been stepped to the next lower voltage tap. This process continues for each succeeding scanning aperture and repeats for each 360 of rotation of the scanning disc 20.

The operation of the scanning system is independent of the speed of the scanning disc provided this speed is below the limits imposed by the masses of the moving mechanical parts. High scanning speeds are not ordinarily required in a plasma scanner. The scanner shown will easily operate at 60 rpm. For slower scanning speeds a long persistent phosphor should be used. For the slowest scanning speeds, or for a single rotation of the scanning disc, a storage type of cathode ray tube may be used.

I claim:

1. Apparatus for analyzing plasma streams comprising: plasma stream producing means including a cathode where the plasma stream originates, a ring anode having an opening through which the plasma stream passes, and a source of direct potential having its positive terminal connected to said anode and its negative terminal to said cathode; a scanning disc, made of a plasma resistant insulating material, positioned to receive said plasma stream after passage through said ring anode, said scanning disc being rotatable about an axis parallel to the axis of said plasma stream and lying outside said stream and having a series of scanning apertures at equal angular spacings and uniformly decreasing distances from the center of rotation of the disc for scanning along parallel arcuate lines across the cross section of said plasma stream; a collector electrode positioned behind said scanning disc op posite the area on which said plasma stream impinges and coextensive with said area for receiving the plasma particles passing through the apertures of said disc; a resistor having a value much lower than the resistance of said plasma stream and a second source of direct potential connected in series between said collector electrode and said anode; and an indicator connected across said resistor.

2. Apparatus as claimed in claim 1 in which a bafile plate made of a plasma resistant insulating material is situated between said scanning disc and said ring anode, said plate having an opening concentric with and commensurate with the opening in said ring anode.

3. Apparatus as claimed in claim 2 in which means are provided for immersing said plasma stream in a constant magnetic field parallel to the stream axis.

4. Apparatus as claimed in claim 1 in which said indicator comprises a cathode ray tube, means for controlling the intensity of the beam of said tube in roportion to the voltage drop across said resistor, and means coupled to said scanning disc for producing a sweep of the beam of said tube across its screen that is synchronized with the scanning of said scanning disc.

5. Apparatus as claimed in claim 1 in which said second source of direct potential produces a linear sawtooth of voltage having a frequency much higher than the rotational frequency of said scanning disc and in which said indicator comprises a cathode ray tube, means for deriving a signal proportional to the logarithm of the volttage drop across said resistor, means for producing from said logarithmic signal a signal proportional to the first derivative of said logarithmic signal relative to the voltage of said second source, means for controlling the intensity of the beam of said tube in accordance with the lastnamed signal, and means coupled to said scanning disc for producing a sweep of the beam of said tube across its screen that is synchronized with the scanning of said scanning disc.

References Cited UNITED STATES PATENTS RUDOLPH V. ROLINEC, Primary Examiner E. L. STOLARUN, Assistant Examiner US. Cl. X.R. 

